1. Field of the Invention
This invention relates to casting molten steel.
2. Description of the Prior Art
In normal practice, molten steel produced by any of the classic processes, for example, the B.0.F., the Q.B.0.P., or the electric furnace process, usually contains a high level of oxygen. This degrades the steel. To overcome this, the steel is killed by introducing into the molten steel deoxidizing agents, for instance, silicon, in the form of ferro silicon or aluminum or both. This is usually performed in a transfer ladle, at tap.
Following deoxidation treatment, the killed molten steel has a strong affinity for oxygen, which it picks up when exposed to the atmosphere, during pouring from a furnace, or casting into ingot molds, into billets, or into slabs. This results in defects, for example, non-metallic inclusions, in the resulting steel which can reduce the quality of the finished products.
To prevent or to reduce this "oxygen pick-up", various protective methods have been used. One involves shielding open cast steel streams between tundish and mold with ceramic tubes. This has been an established practice for maintaining high quality in continuous casting of large bloom and slab sections. It cannot be applied to smaller bloom and billet sections, however, because of space limitations. An example of this type of process is found in Canadian Pat. No. 1,097,881, Thalmann et al, Mar. 24, 1981.
In another expedient, liquid argon has been poured into ingot molds. The argon evaporates on contact with the molten steel and shields it from the atmosphere as it continues to be poured into the mold. Main drawbacks of this method are that the storage and transfer of equipment is difficult to adapt to the hard working conditions of the pouring floor, and, the cost of argon in relation to the price of normal grades of steel is high.
The inert gas shrouding of strand cast steel has also been described in the article "Gas Shrouding of Strand Cast Steel at Jones & Laughlin Steel Corporation" by Samways, Pollard & Fedenko, Journal of Metals, October 1974 pages 28-34. U.S. patents relating to this method are U.S. Pat. No. 3,908,734, Sept. 30, 1975, U.S. Pat. No. 3,963,224, June 15, 1976, and U.S. Pat. No. 4,023,614, May 17, 1977, all to Pollard.
Another method uses liquid nitrogen to form a shroud about the molten steel as it is teemed into a continuous casting machine. This is described in the brochure entitled "Conspal Surface Protection", published by Concast AG, Zurich, Switzerland, March 1977 and in U.S. Pat. No. 4,178,980 (1979), L'Air Liquide. In general, liquid nitrogen has provided a degree of protection which gives some improvement over other methods. But, handling this substance under the hard conditions of the pouring floor makes it difficult to provide continuity of flow, during the operation. Also, nitrogen has has a density close to that of air, reducing its ability to displace air effectively. Moreover, nitrogen inerting is not practicable for grades of steel where nitride formation is undesirable.
The disclosure of the publications and patents mentioned are hereby incorporated by reference.